One of the reasons why diamonds are preferred as gems lies in their characteristic brilliancy. This brilliancy arises from the fact that light incident on a diamond is reflected internally by its cut facets, spectrally dispersed, and collected on the side of the crown, i.e., on the surface side. The diamond exhibits brilliancy in various colors because of the spectral dispersion inside it. Therefore, the degree of the brilliancy and the beautifulness of a diamond depend much on the quality of cutting and the proportions.
A diamond that is ideally brilliant cut for obtaining the highest brilliance is well proportioned, and the superfluous portions have been removed from it. The cut facets are oriented in correct directions. Most of the light incident on the diamond is regularly reflected internally by the cut facets and collected on the side of the crown. Consequently, the greatest and most splendid and varied brilliancy is shown.
On the other hand, a diamond that is roughly cut to derive the maximum carat is ill proportioned and has superfluous portions. The cut facets are oriented in random directions. Therefore, the incident light is not regularly reflected internally. Only a small portion of the light is collected on the side of the crown. For these reasons, the brilliancy is poor, and the jewel lacks splendor. In the case of an especially poorly cut diamond, the incident light may directly pass to the rear side, or the pavilion.
Normally, ordinary buyers observe diamonds with the naked eye in shops. The diamonds appear to exhibit almost identical brilliance irrespective of the quality of the cutting by the effects of the illumination. It is very difficult for those who have no expert knowledge to see whether the diamond has been cut satisfactorily.
Accordingly, the present situation is that manufacture and sale of diamonds rely mostly on their carats because carats can directly affect the prices. The quality of cutting and the proportions tends to be less adopted in evaluating gems since these factors cannot be easily judged as mentioned above.
A simple implement for observing the brilliance of diamonds has been proposed in Japanese Utility Model Laid-Open No. 109041/1985. In particular, a diamond of interest is placed between a magnifying glass and a light source. A red disk which is centrally provided with a hole is mounted to the side of the objective lens of the magnifying glass. The light from the light source is directed to the disk so that the light may be reflected to the diamond. If a large amount of red light is observed, then the gem is taken to be excellent in brilliance.
However, the fringes of the diamond observed with this implement are quite simple, i.e., composed of white, black, and one kind of red. Shining portions are red in color. Those portions which are not shining are white in color. It is impossible to know the nature of black portions. Usually, changes in the intensity of light should produce changes in the density of the fringes, but the fringes observed with this implement are uniform in density. Furthermore, no three-dimensional characteristics are observed. In this way, this known implement does not permit one to judge the brilliance of diamonds sufficiently precisely.
A photograph is attached to a written statement of an expert opinion on a diamond. This photograph is taken by a camera placed above the diamond while illuminating it from below. In the produced fringes, the brightest and least bright portions appear black to the naked eye, while the other portions appear whitish. This photograph has the following disadvantages.
(1) Those portions which should appear brightest are black like in a negative film of a black-and-white photograph.
(2) Those portions which should be brightest and those portions which are least bright are equally black in the resulting image. Therefore, it is completely impossible to know which portions are really bright.
(3) The white portions in the photograph should have contain portions of varied degrees of brightness. However, these varied degrees of brightness produce little changes in the density in this photograph. Hence, almost no difference in brightness can be seen in the white portions. This also totally deprives the fringes of three-dimensionality.
In this way, this photograph is very difficult for laymen to understand though this situation may not apply to experts.
As described previously, the brightness specific to diamonds depends not only on the amount of reflection of light inside the gems but also on emission of various colors due to spectral dispersion. Nonetheless, it is impossible for the unaided eye to see the extent of the variedness of the colors of the emitted light. Also, it has been impossible to pick up varied colors of light emitted from a diamond, using the above-described conventional implement or photography method.
Where a diamond is photographed in a conventional manner to prepare a catalog or the like, light is directed to the diamond from the side of the camera. In this photography, it is almost impossible to pick up the above-described varied colors from the diamond. In this manner, any photography or implement which makes it possible to catch various colors of light emitted by a diamond is not known.